The present invention relates in general to solenoid-operated fluid control valves of the type described in the ""425 application and the ""033 and ""947 Patents, which may be used in precision fluid flow regulation systems, such as those that require precise control of the rate of fluid flow, including but not limited to pneumatic and hydraulic regulation. The invention is particularly directed to a modification of the configuration of the magnetic pole piece, that obviates the need for an alignment and support element of non-magnetic material, thereby reducing the complexity and cost of manufacturing.
As described in the above-referenced ""425 application and the ""033 and ""947 patents, precision fluid flow control devices commonly employ a solenoid-operated valve for controlling fluid flow substantially proportional to the current applied to the solenoid. It is also desirable that hysteresis in the flow rate versus control current characteristic (which creates an undesirable dead band in the operation of the valve) be maintained within some minimum value. A standard practice for reducing hysteresis has been to physically support the solenoid""s moveable armature within the bore of its surrounding drive coil by means of low friction bearings, such as Teflon rings. However, even with the use of such a low friction material, there is still significant xe2x80x98dead bandxe2x80x99 current (e.g. on the order of forty-five milliamps), which limits the operational precision of the valve.
One proposal to deal with this physical contact-created hysteresis problem is to remove the armature support mechanism from within the bore of the solenoid coil (where the unwanted friction of the armature support bearings is encountered) to an end portion of the coil, and to support the armature for movement within the bore by means of a spring mechanism located outside of the solenoid coil. An example of such a valve configuration is described in the U.S. Pat. to Everett, No. 4,463,332, issued Jul. 31, 1984.
According to this patented design, the valve is attached to one end of an armature assembly supported for axial movement within the cylindrical bore of the solenoid coil and having a permanent ring magnet surrounding the solenoid. One end of the solenoid contains a ring and spring armature support assembly, located substantially outside the (high flux density) solenoid bore, and whose position can be changed, so as to adjust the axial magnetic flux gap within the bore and thereby the force applied to the valve.
Unfortunately, this type of support structure requires a magnetic flux booster component which, in the patented design, is a permanent magnet. Namely, even though the objective of the Everett design is to adjust magnetic permeance and maintain linearity, the overall solenoid structure and individual parts of the solenoid, particularly the ring spring armature assembly (which itself is a complicated brazed part), and the use of a permanent booster magnet, are complex and not easily manufacturable with low cost machining and assembly techniques, resulting in a high price tag per unit. In another prior art configuration, described in the U.S. Pat. to Nielsen, No. 4,635,683, the movable armature is placed outside the bore by means of a plurality of spiral spring-shaped bearings adjacent to opposite ends of the solenoid structure.
Advantageously, the linear motion proportional solenoid assembly described in U.S. Pat. No. 4,954,799 (hereinafter referred to as the ""799 patent) entitled: xe2x80x9cProportional Electropneumatic Solenoid-Controlled Valve,xe2x80x9d improves on the above designs by using a pair of thin, highly flexible annular cantilever-configured suspension springs, to support a moveable armature within the bore of solenoid, such that the moveable armature is intimately coupled with its generated electromagnetic field (thereby eliminating the need for a permanent magnet as in the Everett design).
In order to make the force imparted to the movable armature substantially constant, irrespective of the magnitude of an axial air gap between the armature and an adjacent magnetic pole piece, the device detailed in the ""799 Patent places an auxiliary cylindrical pole piece region adjacent to the axial air gap. This auxiliary cylindrical pole piece region has a varying thickness in the axial direction, which serves to xe2x80x98shuntxe2x80x99 a portion of the magnetic flux that normally passes across the axial gap between the armature assembly and the pole piece element to a path of low reluctance. By shunting the flux away from what would otherwise be a high reluctance axial path through a low reluctance path, the auxiliary pole piece region effectively xe2x80x98linearizesxe2x80x99, the displacement vs. current characteristic over a prescribed range.
The proportional solenoid structure described in the ""298 Patent and diagrammatically shown in FIGS. 1 and 2, reduces the structural and manufacturing complexity of the implementation of the structure described in the ""799 Patent by locating a moveable, ferromagnetic (or simply magnetic) armature 10 adjacent to one end of a fixed pole piece 12 made of ferromagnetic (magnetic) material that protrudes outside a solenoid coil bore 14, and configuring this moveable armature 10 to provide two, relatively low reluctance magnetic flux paths 21 and 22. (For a description of additional details of the solenoid-actuated valve structure shown in FIGS. 1 and 2, attention may be directed to the ""298 Patent, proper.)
Now even though the proportional solenoid structure described in the ""298 Patent operates extremely well in relatively small and larger sized hardware configurations, for very small (e.g., micro-valve) applications and using reasonable priced industry standard materials, it is possible for one or more components of the assembly may become distorted, particularly those parts that are very small and dimensionally thin (such as the moveable armature""s support springs). Namely, for very small dimension applications, what would otherwise be a negligible axial magnetic flux component accompanying the dominant radial flux component bridging the variable geometry radial air gap 32 between the saturated tapered rim portion 34 of the moveable armature 10 and the inwardly projecting tapered portion 36 of the solenoid assembly housing 30 becomes significant.
In particular, the non-radially directed magnetic flux in the variable geometry air gap 32 can overcome the mechanical rigidity of the material (e.g., beryllium copper) of the armature support springs 41 and 42, and cause the springs to warp or twist from their intended shape, and deviate from their intended axial cantilever axial flexing.
This unwanted distortion of the armature support springs is particularly likely where there are nontrivial departures from dimensional tolerances in the manufacturing of the parts of the solenoid assembly. Because of the variable geometry gap inherently tends to provide some degree of play between the armature and the housing, distortion of the armature support springs can cause an unbalanced physical engagement of the tapered rim portion of the moveable armature with the inwardly projecting tapered portion of the housing, thereby preventing proper operation of the proportional solenoid assembly.
The invention disclosed in the ""425 application and the ""033 and ""947 Patents (diagrammatically illustrated in FIGS. 3 and 4 as comprising a valve unit 100 coupled with a valve-control solenoid unit 200) remedies this component distortion problem by modifying the configuration of the moveable armature to eliminate the variable geometry annular air gap between the radially projecting, tapered rim portion of the moveable armature and the inwardly projecting tapered portion of the solenoid assembly housing, while still retaining their flux control functionality. (For a description of additional details of the solenoid-actuated valve structure shown in FIGS. 3 and 4, attention may be directed to the ""033 and ""947 Patents, proper.)
While the solenoid structure of the ""033 and ""947 Patents and the ""425 application is very effective in eliminating the variable geometry annular air gap between the radially projecting, tapered rim portion of the moveable armature and the inwardly projecting tapered portion of the solenoid assembly housing, it uses a non-ferromagnetic element (in the form of a step-shaped step-shaped annular support ring 206), to confine the magnetic flux path between the lower end of the magnetic pole piece 220 and the movable armature 170, and to maintain all of the elements in coaxial alignment.
Installation of a non-ferromagnetic element has a number of drawbacks, particularly with regard to manufacturing complexity and incorporating metallurgically diverse materials in the overall magnetic flux formation and confinement path. In the architecture of the ""033 and ""947 Patents and the ""425 application, the entire solenoid structure is constructed in such a way as to isolate the fluid medium from the internal solenoid components by the use of isolation diaphragm. Since the internal volume of the solenoid assembly is not subjected to any fluid pressures, it is possible to design the solenoid in such a way, that the non-magnetic element is fixed in place, thereby providing structural rigidity and coaxial alignment. However, the non-magnetic circuit is essential for the proper operation of the solenoid. In applications where the use of the diaphragm is unacceptable, such as miniature valves or high pressure valves, then the structure of the solenoid has to be changed to resort to other mechanical assembly and manufacturing processes.
If the fluid is allowed inside the solenoid, then this non-magnetic circuit is accomplished by placing a non-magnetic element and rigidly attaching it to a magnetic element above and below, by means of manufacturing processes that are costly and tedious. Techniques that are used for this purposes include but not limited to: swaging, electron beam welding, laser welding, brazing etc. In most instances, a secondary machining operation would be necessary, in order to achieve coaxial alignment of the magnetic and non-magnetic parts.
Even with successful (and costly) assembly of the components, there still remains the issue of employing different metals with different coefficients of thermal expansion.
One proposal to address these and other problems associated with the use of non-ferromagnetic materials for magnetic flux path control in the solenoid assembly is described in the U.S. Pat. to Nippert et al, No. 5,986,530. The solution offered by the Nippert el. al Patent, which still requires the use of a non-ferromagnetic material for magnetic flux path control, is to form an annular dual-tapered groove in the external surface of a single piece of ferromagnetic stock, which serves as the solenoid housing. By forming the housing from a single piece of ferromagnetic material, the patentees seek to avoid concentricity (alignment) problems of conventional solenoid assemblies. The dual tapered groove is then filled with a non-ferromagnetic material, as by gas tungsten brazing/welding, and an axial bore is formed through the housing so as to intersect the groove. What results is a structure having two separate tapered ferromagnetic pieces joined by a tapered sleeve of non-ferromagnetic material. The non-ferromagnetic sleeve serves as a bearing for supporting an armature and associated armature pin (pole piece) within the housing.
Now even though Nippert et al seek to define the intended flux path and achieve component alignment by referencing the overall assembly process to a single piece of ferromagnetic stock, their proposed assembly scheme still requires the use of a non-ferromagnetic element, drawbacks of which are enumerated above.
In accordance with the present invention, the use of a non-magnetic material in the magnetic flux flow path of a proportional solenoid architecture is eliminated, by configuring the ferromagnetic pole piece to include a main longitudinal axial pole piece portion, and a relatively thin, annular axial pole piece portion that is effectively mechanically solid with the main longitudinal pole piece portion and is configured to provide for fluid leakage containment. The magnetic pole piece further includes a radial portion that is solid with the annular axial portion.
The radial portion of the magnetic pole piece is affixed to both the solenoid""s housing and a valve unit. As such and being solid with the annular axial pole piece portion, it enables the magnetic pole piece to provide support and alignment for the armature relative to the magnetic pole piece, without the need for non-magnetic material for alignment, support or magnetic flux flow path control. The valve unit includes a valve poppet coupled with the axially movable armature, so as to regulate fluid flow between a fluid input port and a fluid exit port of the valve unit.
Since the integral pole piece and support architecture of the invention does not require a non-magnetic material in the magnetic flux flow path or as part of its support structure, it reduces hardware and manufacturing complexity and cost, particularly the need for welding, associated with solenoid structures that use non-ferromagnetic materials as part of flux path containment and pole piece/armature alignment.